1. Technical Field
The present invention relates to a process for the formation of patterns (e.g. holes), in a photosensitive resin layer or photoresist, as well as to an apparatus for performing this process. The invention more particularly applies to the production of microtip emissive cathode electron sources, which are more particularly used in the fabrication of display means by cathodoluminescence excited by field emission. The invention makes it possible to produce microtip flat screens of a larger size than that of the prior art (e.g. exceeding 14 inches and even microtip flat screens with an area close to 1 m.sup.2).
2. Prior Art
Microtip emissive cathode electron sources and their production processes are described in (1) FR-A-2 593 953 (cf. also EP-A-234 989); (2) U.S. Pat. No. 4,857,161 (corresponding to document (1); (3) FR-A-2 663 462 (cf. also EP-A-461 990 and U.S. Pat. No. 5,194,780); (4) FR-A-2 687 839 (cf. also EP-A-558 393 and U.S. patent application Ser. No. 08/022,935 by Leroux et al. of 26.2.1993) to which reference can be made. To facilitate understanding of the technical problem solved by the present invention, a description will now be given of a known example of a process for the production of a microtip emissive cathode electron source, with reference to the attached FIGS. 1.
FIG. 1 shows an existing structure comprising a substrate 2, surmounted by an insulator 4, a system constituted by cathode conductors 6, a resistive layer 7 and grids 8, the conductors 6 and grids 8 being superimposed in crossed form with an intermediate insulator 10 and a mask 12 (e.g. nickel layer) deposited on the surface to serve as a mask during the microtip production operations. This nickel layer 12, the grids 8 and the insulator 10 are perforated by holes 14, on the bottom of which will be subsequently deposited the microtips made from a conductor metal electrically connected to cathode conductors 6 through the resistive layer 7.
The production of the microtips will now be explained relative to FIG. 2. Firstly in exemplified manner a molybdenum layer 16 is deposited on the complete structure. The layer 16 has a thickness of approximately 1.8 .mu.m. It is deposited under normal incidence with respect to the surface of the structure. This deposition procedure makes it possible to obtain molybdenum cones 18 located in holes 14 and having a height of 1.2 to 1.5 .mu.m. These cones form the electron emitting microtips. This is followed by the selective dissolving of the nickel layer 12 using an electrochemical process so as to free, as can be seen in FIG. 3, the perforated grids 8, which are e.g. of niobium, and bring about the appearance of the electron emitting microtips 18.
To within a few technological variants, the thus described known process with reference to FIGS. 1, 2 and 3 is one of those used for producing microtips of microtip emissive cathode electron sources. In order that the size and positioning of the microtips 18 are correct, it is obviously necessary to perfectly control the size of the holes made in the grids 8 and in the insulator 10. It is a question of producing on all the surfaces which are to receive microtips, holes with a mean diameter of e.g. 1.3 .mu.m or less. The methods used at present for producing these holes use photolithography processes employing the direct projection or photorepetition of an elementary pattern reproduced on all these surfaces.
In the case of large electron sources, larger than 14 inches (approximately 35 cm), the process of producing this pattern of holes rapidly become constrictive. The direct projection requires the production of a large scale size mask having submicron patterns. These patterns are generally formed from thin film metal deposited on a glass or silica substrate. It is difficult to produce this mask in excess of 14 inches diagonal using conventional microelectronics methods.
With regards to photorepetition, use is made of a small mask, the size being determined by the resolution of the patterns used. For a resolution of 1 .mu.m, use is made, for e.g., of a 20 to 50 mm side length mask, which makes it necessary to repeat a large number of times the irradiation operation necessary for photolithography in order to cover the complete surface of the electron source.
These two methods (one using direct projection and the other photorepetition) are consequently difficult to apply to the production of large electron sources.
Another method referred to in (1) "Large scale field emitter array patterning for flat panel displays using laser interference lithography", J. P. SPALLAS, R. D. BOYD, J. A. BRITTEN, IVMC Conference 1995; (2) "Arrays of gated filed emitter cones having 0.32 .mu.m tip to tip spacing", C. O. BOLZER, C. T. HARRIS, S. RABE, D. RATHMAN, J. Vac. Sci. Technol. B12(2), March/April 1994, p 629; (3) "Achromatic interferometric lithography for 100 nm period gratings and grids", T. A. SAVAS, S. N. SHAH, H. I. SMITH, J. Vac. Sci. Technol. B13(6), November/December 1995, p 2732; (4) "Large area, flee-standing gratings for atom interferometry produced using holographic lithography", J. M. CARTER, D. B. OLSTER, M. L. SCHATTENBURG, A. YEN, H. I. SMITH, J. Vac. Sci. Technol. B10(6), November/December 1992, p 2909 consists of carrying out photolithography with the aid of laser interferences in order to irradiate a photosensitive resin. This method consists of creating a system of light fringes on the surface of the resin by bringing about interference between two laser beams forming a given angle and coming from the same laser, followed by the recommencement of said operation after turning the substrate by 90.degree.. This gives an irradiated area in grid form, whose intersections are twice more irradiated than the remainder. After development of the resin and in the case where a positive resin is used, there is consequently an array of holes, whose period (distance between two holes) is equal to the distance between two light fringes.
Among recent publications referring to this process, reference can be made to that of J. P. SPALLAS et al., Lawrence Livermore National Laboratory (IVMC95), which mentions the production of an array of 10.sup.11 tips, of diameter 330 nm, on a 50 cm.sup.2 substrate (cf. document (5)). However, the authors of this publication remain very discreet concerning the results obtained. Thus, the AFM (atomic force microscope) photographs published are in all cases relative to small substrates of a few cm.sup.2. Reference can e.g. made to an array of photosensitive resin tips of diameter 330 nm, spaced by 670 nm and having a height of 250 nm on a 2.5.times.7.5 cm substrate.
With regards to their tests on a 50 cm.sup.2 plate, the experimental conditions are as follows:
krypton laser emitting a radiation of wavelength 413 nm, PA1 resin marketed by Shipley under reference 1400, whose refractive index is close to that of the glass substrate in order to avoid interference, PA1 spreading of the resin in a system known as meniscus coating, PA1 total irradiation time of 4 minutes (2 minutes for each half irradiation). PA1 10.sup.10 holes to be produced, PA1 diameter of holes 1 .mu.m, PA1 surface of one hole 0.785 .mu.m.sup.2, PA1 photosensitivity of the photosensitive resin S=300 mJ/cm.sup.2 (standard value). PA1 elementary light beams are formed, whose axes are parallel, PA1 at least one first relative translation takes place at a constant speed and constant light power of said elementary beams with respect to the photoresist, the elementary beams being focused thereon, so as to irradiate first parallel lines of said photoresist, said speed and said power being chosen in such a way that each first irradiated line receives a first light dose lower than the light dose necessary for the development of said photosensitive resin, PA1 there is a relative rotation by a given angle of all the elementary light beams with respect to the photoresist, PA1 there is at least one second relative translation, at constant speed and constant light power, of said elementary beams with respect to the photoresist, the elementary beams being focused on the latter, so as to irradiate second parallel lines of said photoresist, said speed and said light power being chosen in such a way that each second irradiated line receives a second light dose equal to the difference between the light dose necessary for the development and the first dose, so that only the intersections of the first and second lines receive the dose necessary for the development and PA1 the photosensitive resin is developed, patterns consequently being formed at said intersections. PA1 a light source able to transmit a light beam having a constant power, PA1 means for forming said focused, elementary light beams from the constant power light beam and PA1 means for the relative translation and rotation of the means for forming said elementary light beams with respect to the photo resist, said relative translation and rotation means being provided to carry out said first and second relative translations and said relative rotation. PA1 an array of optical fibres, said optical fibres having first ends, whose axes are parallel to one another, and second ends, which are optically coupled to the light source, so as to form elementary light beams respectively in the optical fibres and PA1 an array of optical focusing means optically coupled respectively to the first ends of the optical fibres and able to focus the elementary light beams onto the photoresist. PA1 a structure is formed having cathode conductors on a substrate, an electrically insulating layer and grids forming an angle with the cathode conductors, PA1 in the areas where the grids cross the cathode conductors, holes are formed through the grids and the insulating layer and PA1 electron emitting material microtips are formed in these holes on the cathode conductors, said process being characterized in that said holes are obtained by forming a positive photoresist at least in said areas, at the surface of the structure, forming holes in the photoresist in accordance with the pattern formation process according to the invention, and etching the grids and insulating layer through said holes formed in the photoresist.
Thus, no information is given on the uniformity defects, which would probably appear as a result of imperfections of the profile of the laser beams. No information is given on the collimation of the laser beams. Moreover, this method significantly restricts the choice of the configuration and distribution of the holes. Thus, the period P of the array, i.e. the distance between two successive holes, is directly linked with the wavelength .lambda. of the laser used and the angle .theta. between the two beams from said laser, which are made to interfere in order to create the system of light fringes, so that it is possible to write: P=.lambda./(2 sin .theta.). Thus, for a given laser wavelength .lambda. and given angle .theta., it would only be possible to obtain an array of holes with a fixed spacing. The size of the holes is determined by the width of the interference fringes.
The present invention avoids all the aforementioned problems. It makes it possible to produce a mask for the formation of holes corresponding to electron emitters in a much simpler manner than the aforementioned prior art methods. It relates to a very simple process for the formation of holes or other patterns in a photoresist, as well as to an apparatus for performing this process. The invention makes it possible to form holes with a diameter of or below 1 .mu.m over small or large surfaces. The invention makes it possible to irradiate a photoresist with the aid of a laser beam without light interferences.
Once this photosensitive resin has been developed (i.e. after dissolving the irradiated areas), it can be used as a mask for forming patterns, such as e.g. holes for microtip electron sources, in a structure on which the photoresist is located.
For example, considering the structure described with reference to FIGS. 1 to 3, the photoresist can serve, after development, for the etching of the grids 8 and the intermediate insulator 10.
One aim of the present invention is to propose a process and an apparatus for producing micrometric or submicrometric patterns in intaglio or relief in a photoresist.
The present invention also aims at a process for the formation of patterns in an intaglio, e.g. holes, or in relief, by the laser photolithography of a photoresist and the etching of one or a plurality of layers placed beneath said photosensitive resin.
Another aim of the invention is to propose a process which is simple to perform and which makes it possible to treat substrates having a photoresist with a large area of e.g. 1 m.sup.2.
The authors of the present invention have attempted to evaluate the possibilities of photolithography of holes by direct laser recording in the existing state of the art with respect to lasers, (without light interferences). The technical problems which arise are described hereinafter.
The staring hypothesis is still the same, the aim being to e.g. produce 10.sup.4 holes per mm.sup.2 on a substrate of approximately 1 m.sup.2. Therefore the initial parameters are as follows:
From this is deduced the light dose D necessary for the irradiation of a hole, i.e. D=2.4 nW/s.
We will now consider the limitations of existing laser systems, namely laser diodes, microlasers, gas lasers and YAG solid lasers.
Laser diodes operate at frequencies of approximately 1 MHz. They supply a peak power of a few watts, which could satisfy the application considered here, but it is presently impossible in the state of the art to focus the laser beam onto such small surfaces.
With regards to microlasers, the presently usable operating frequencies are approximately 10 KHz and there is no microlaser able to emit in the ultraviolet range. Thus, microlasers are unusable for the application considered here.
With regards to gas lasers in the ultraviolet field, existing excimer lasers operate at a few W/cm.sup.2 in the continuous or burst mode, but in the pulsed mode the maximum frequencies are a few dozen hertz. Thus, these gas layers do not satisfy the application considered here.
In the case of YAG solid lasers, by tripling the frequency thereof, it is possible to operate at approximately 355 nm. In the continuous mode a few watts are obtained without difficulty. However, such solid lasers are unusable for the application considered here in the pulsed mode, because the maximum frequency is approximately 10 kHz.
Consideration could be given to recording by sequential irradiation hole by hole. In this case the aim is to irradiate the holes and nothing else.
One solution e.g. consists of using a YAG laser and overcoming the frequency problem by subdividing the light beam produced into multiple elementary beams (e.g. 1000). For this purpose use is made of a bundle of optical fibres or an image multiplier system coupled to a microlens strip or linear array in order to refocus the elementary beams at the exit. These microlenses exist in integrated strip form and it is possible to arrange a microlens approximately every 100 .mu.m, which leads to a 10 cm long apparatus. The problem which then has to be solved is the coordination of the displacement and alignment of the system obtained with the modulation of the laser beam.
A mechanical displacement in two perpendicular directions x and y is inconceivable due to the speed necessary. Information might be obtainable from rotary mirror systems used in photocopiers for deflecting the elementary beams of a pulsed YAG laser and avoid the displacement on each occasion of the strip. With such a system, it would be necessary to photorepeat a sequence of laser flashes with stoppage of the laser and alignment between each sequence.
Another solution would consist of working with a YAG laser in the continuous mode and displacing the elementary beams sufficiently rapidly between the holes so as not to irradiate the resin. This solution avoids the synchronization of the displacement and alignment with the flash frequency of the pulsed YAG laser. A rapid calculation makes it possible to estimate the necessary displacement speed. Assuming the use of a 1 W laser permitting the formation of 1000 elementary beams of 1 mW, 2.4 nJ per hole would be needed and consequently each hole would have to be irradiated for 2.4.times.10.sup.-6 seconds. The total irradiation time for 1 m is consequently approximately 25 seconds (because there are 10.sup.10 holes), which is completely reasonable. However, the major problem still remains of the displacement between the holes.
If it is borne in mind that it is necessary to remain half the time on the photosensitive resin between the holes to prevent the irradiation thereof and that the interval is approximately 4 .mu.m, it is necessary to displace the elementary beams or plates carrying the photosensitive resin at a speed of approximately 4 m/s and on each occasion it must be displaced by a few micrometers. Unfortunately, in the state of the art, the best piezoelectric displacements which would make it possible to displace the plate by a few micrometers do not make it possible to obtain speeds exceeding a few cm/s. Thus, this constitutes the limitation in that it is difficult or even impossible to displace a 1 m.sup.2 plate or a 10 cm strip by a few .mu.m with a total travel close to 1 m and at a speed of 4 m/s without stopping.
The present invention avoids having to solve this problem. For this purpose, the invention uses recording by continuous irradiation. The same type of system as hereinbefore is used with an image multiplier (.times.1000) and a continuous YAG laser. The method used consists of merely continuously moving the 1000 elementary beams at a continuous speed over the entire plate to be treated, so as to obtain a succession of parallel lines on the photosensitive resin.
The major difference compared with what has been stated hereinbefore is that there is no longer a stoppage at each hole and there is consequently no longer any need for a piezoelectric displacement. Each line is irradiated with half the dose necessary for development. The plate is then turned by 90.degree. and the same line pattern is reproduced. The figure obtained is a grid, whose intersections have received a light dose permitting the development of the holes at these intersections.
For example, choice is made of a laser which is half as powerful as hereinbefore (1/2 W) in order to irradiate the lines at half the dose necessary for development and the plate is moved at the same speed of 4 m/s, which is completely conceivable in that there is a continuous displacement of the plate. The total treatment time is approximately 1 minute or a few minutes, if account is taken of vertical displacements and overpasses at the end of the plate. It is possible to choose a less powerful laser or a strip having more than 1000 optical fibres. The travel speed of the plate in front of the optical fibres is slower in the latter case and a less rapid motorization system is then chosen.
Special reference is made to the binary character of the process used in the invention, namely at each intersection a hole is obtained and there are no holes elsewhere. Thus, the aim is not to modulate the power of the laser in order to obtain sloping resin patterns, as in the case of documents (1) "Preshaping photoresist for refractive microlens fabrication", T. R Jay et al., SPIE vol. 1992, Miniature and Micro-Optics and Micromechanics, 1993, pp 275 to 282; (2) "One-step 3D shaping using a gray-tone mask for optical and microelectronic applications", Y. Oppliger et al., Microelectronic Engineering 23, 1994, pp 449 to 454; (3)"Fabrication of relief-topographic surfaces with a one-step UV-lithographic process", H. J. Quenzer et al., pp 163 to 172; (4) "Continuous-relief microptical elements fabricated by laser beam writing", G.
Przyrembel, pp 219 to 228; (5) "Generation of relief-type surface topographies for integrated microoptical elements", J. Wengelink et al., pp 209 to 217; (6) "Laser beam lithographed micro-Fresnel lenses", M. Haruna et al., Applied Optics, vol. 29, No. 34, Dec. 1, 1990, pp 5120 to 5126; and (7) "Lighographie tridimensionnelle par exposition a niveaux de gris", Japon Optoelectronique, No. 17, December 1993, January-Febuary 1994, p 16.